1. Field of the Invention
This invention relates to masks for use in x-ray photolithography and to processes for manufacturing such masks. This invention also relates to masks used when bringing up alignment apparatus.
2. Description of the Prior Art
It is known in the art to use x-rays in photolithographic processes for manufacturing integrated circuits. In such processes a mask is used which selectively blocks x-rays. An example of such a mask is mask 7 illustrated in FIG. 1. Mask 7 includes an x-ray transparent membrane 1, typically boron nitride, formed on a silicon ring 2. Silicon ring 2 is bonded through a ring of boron nitride to a pyrex ring 3 which serves as a support structure for mask 7. A polyimide layer 4 is formed on boron nitride membrane 1 to provide enhanced mechanical support for membrane 1. A patterned gold layer 6 (which is x-ray opaque) and a similarly patterned tantalum layer 5 (which bonds to both gold and polyimide) is formed on polyimide layer 4. Gold layer 6 selectively blocks x-rays during photolithographic processes for manufacturing integrated circuits. In a typical prior art mask, boron nitride membrane 1 is between 3 and 5 microns thick and polyimide layer 4 is about 2 microns thick.
Although mask 7 of FIG. 1 performs well in x-ray photolithographic processes, it is easily damaged. Accordingly, it is desirable to provide a mask having enhanced mechanical strength. Such a mask is useful when first bringing up alignment apparatus. As used in this specification, "bringing up alignment apparatus" means turning on the alignment apparatus for the first time, i.e. after receiving the apparatus from the manufacturer. As is known in the art, alignment apparatus is used during the fabrication of semiconductor devices to selectively expose photoresist on a set of wafers through a mask. Each wafer within the set is placed on a support chuck which moves with respect to the mask when it is desired to align the mask and the wafer. When first bringing up alignment apparatus, it is possible for a wafer to strike and damage the mask before fine mechanical adjustments have been made to the alignment apparatus.